Preload fix (#34)

* Don't try to preload if monarch is busy
* Added test for #32

monarch/monarch.lua

@@ -621,6 +621,11 @@ end
 -- @param id (string|hash) - Id of the screen to preload
 -- @param cb (function) - Optional callback to invoke when screen is loaded
 function M.preload(id, cb)
+	if M.is_busy() then
+		log("preload() monarch is busy, ignoring request")
+		return false
+	end
+
 	assert(id, "You must provide a screen id")
 	id = tohash(id)
 	assert(screens[id], ("There is no screen registered with id %s"):format(tostring(id)))

test/test_monarch.lua

@@ -238,6 +238,14 @@ return function()
 			assert(wait_until_not_busy())
 		end)
 
+		it("should ignore any preload calls while busy", function()
+			monarch.show(TRANSITION1)
+			-- previously a call to preload() while also showing a screen would
+			-- lock up monarch. See issue #32
+			monarch.preload(TRANSITION1)
+			assert(wait_until_shown(TRANSITION1), "Transition1 was never shown")
+		end)
+
 		it("should be able to notify listeners of navigation events", function()
 			local URL1 = msg.url(screens_instances[hash("/listener1")])
 			local URL2 = msg.url(screens_instances[hash("/listener2")])